An organic electroluminescence display device may be a passive matrix organic light emitting device (PMOLED), or an active matrix organic light emitting device (AMOLED) For a display device having a large area and a high resolution is required, the development of the AMOLED is desired.
An electroluminescence device is a spontaneous light emitting device for emitting light by electrically exciting a fluorescent organic compound. The electroluminescence display device can be driven at a low voltage and can be fabricated as a thin type. Also, the electroluminescence display device is being considered for use due to a wide optical viewing angle, a fast response speed, and other attributes.
In a electroluminescence device, electrons move to a light emitting layer through a cathode through an electron transfer layer. Holes move to the light emitting layer through an anode through another transfer layer. The electrons and the holes are coupled to each other in the light emitting layer, which is an organic material to form an exciton. As the exciton transitions to a low energy state, light is generated.
The generated light has different colors according to the selection of the organic material. The exciton can generate natural colors by using organic materials that emit R, G, and B colors.
An organic electroluminescence display device may further be categorized as a single-layer and a multi-layer structure. The single-layer device has a structure such that one light emitting layer is formed between an anode and a cathode as an organic layer, and the multi-layer has a structure such that a plurality of organic layers including a light emitting layer are formed between the anode and the cathode.
An organic electroluminescence display device of the multi-layer structure is being widely used because a driving voltage can be lowered since carriers are not directly injected into the light emitting layer.
A related art organic electroluminescence display device with a multi-layer structure is shown in FIG. 1.
The device comprises an anode 102, a cathode 101, and an organic electroluminescence layer 110 formed between the anode and the cathode.
The anode 102 is mainly formed of a transparent electrode such as an indium tin oxide (ITO). The cathode 101 is formed of a metal thin film such as Al, and reflects light generated at a light emitting layer.
Holes are supplied to a light emitting layer 104 through the anode 102, and electrons are supplied to the light emitting layer 104 through the cathode 101.
The organic electroluminescence layer 110 comprises the light emitting layer 104, an electron transfer layer 103, and a hole transfer layer 105. The electron transfer layer 103 is formed between the light emitting layer 104 and the cathode 101, and the hole transfer layer 105 is formed between the light emitting layer 104 and the anode 102.
The organic electroluminescence layer 110 is formed on a substrate 107 such as a transparent glass. On the substrate, a unit pixel having a matrix arrangement is formed. Also, at each unit pixel, an organic electroluminescence device having the above structure is formed. The organic electroluminescence display device having the multi-layer structure may comprise much more organic layers, and may further comprise an electron injection layer and a hole injection layer to lower a driving voltage.
A circuit diagram of the organic electroluminescence display device is shown in FIG. 2. M×N unit pixels are formed on an array substrate, and the unit pixel has a matrix arrangement. Each unit pixel 210 defined by the gate line 212 and data line 214 comprises a switching transistor 230, a driving transistor 240, a capacitor 220, and an organic electroluminescence display device 250 for receiving a signal from the driving transistor 240.
A gate electrode of the driving transistor 240 is turned on/off by the switching transistor 230, and the driving transistor 240 is thereby controller. The gate electrode of the driving transistor 240 is connected to a drain electrode of the switching transistor 230.
A source electrode of the driving transistor 240 is connected to a first power supply terminal Vdd of a first power line 216, and a drain electrode of the driving transistor 240 is connected to an anode of the organic electroluminescence device 250. Also, a cathode of the organic electroluminescence device 250 is connected to a second power supply terminal Vss. The organic electroluminescence device 250 is provided with at least one organic layer including an organic light emitting layer.
A plan view of a unit pixel of the organic electroluminescence display device is shown in FIG. 3. The unit pixel of the organic electroluminescence display device is defined by a gate line 301 and a data line 302 perpendicular to the gate line 301. At least one driving transistor 360 and at least one switching transistor 350 are formed in the unit pixel. The driving transistor 360 is controlled by the switching transistor 350.
A first power line 303 parallel with the data line 302 for applying a driving signal to the driving transistor 360 is formed at the unit pixel.
The switching transistor 350 is provided with a first active layer 304a constituting a channel thereof, a source electrode 302a, a drain electrode 310, and a gate electrode 301a. 
The first active layer 304a is extended to be overlapped with the first power line 303, thereby forming one electrode 304b of a storage capacitor. The source electrode 302a is diverged from the data line 302 and is connected to the first active layer 304a through a contact hole. The drain electrode 310 is connected to the first active layer 304a through a contact hole, and one end thereof is connected to a gate electrode 306 of the driving transistor 360 through a contact hole. The gate electrode 301a is diverged from the gate line 301 and supplies a scan signal to the switching device.
The unit pixel of the organic electroluminescence display device is further provided with the driving transistor 360 for driving an organic electroluminescence layer constituting a pixel. The driving transistor 360 comprises a source electrode 303a diverged from the first power line 303, a second active layer 305, a first electrode 307 of the organic electroluminescence device, and a gate electrode 306.
The source electrode 303a is connected to the second active layer 305 through a connection pattern 309 and a contact hole. The first electrode 307 of the organic electroluminescence device serves as a drain electrode of the driving transistor, and is connected to the second active layer 305 through a contact hole. Also, the gate electrode 306 is connected to the drain electrode 310 of the switching transistor 350 and is controlled by the switching transistor 350.
When a scan signal is applied to the gate electrode 306 by the switching transistor, a channel of the second active layer 305 is opened and a driving signal is introduced into the second active layer 305 through the first power line 303. Accordingly, the organic electroluminescence layer of the organic electroluminescence device is driven.
The first active layer 304a and the second active layer 305 are formed on the same layer on the substrate, and the gate line 301 and the gate electrode 306 of the driving transistor are formed on the same line. Also, the first power line 303 is insulated from the active layers and the gate line 303 by an insulating layer, and is formed on an additional layer. The data line 302, the drain electrode 310, and the connection pattern 309 are formed on the same layer. The organic electroluminescence device having the organic electroluminescence layer comprises a first electrode 307 insulated from the data line 302, an organic electroluminescence layer 308 formed on the first electrode 307, and a second electrode (not shown) formed on the organic electroluminescence layer 308.
A sectional structure of the organic electroluminescence display device will be explained with reference to FIGS. 4A and 4B. FIG. 4A is a sectional view taken along line I-I in FIG. 3, which shows the organic electroluminescence display device.
Referring to FIG. 4A, a buffer layer 402 is formed on a substrate 401, and a first active layer 304a and a second active layer 305 are formed on the buffer layer 402. The first active layer 304a is extended to constitute one electrode of a storage capacitor overlapped with the first power line 303.
The active layers 304a and 305a are insulated by a first insulating layer 403, and the gate electrode 301a of the switching transistor 350 and the gate electrode 306 of the driving transistor 360 are formed on the first insulating layer 403.
The gate electrodes 301a and 306 are covered by a second insulating layer 404, and the first power line 303 is formed on the second insulating layer 404.
The first power line 303 is covered by the third insulating layer 405, and the data line 302, the source electrode 302a, the drain electrode 310, and the connection pattern 309 are formed on the third insulating layer 405.
The data line 302, the source electrode 302a, the drain electrode 310, and the connection pattern 309 are insulated by a fourth insulating layer 406, and are protected from outside.
FIG. 4B is a cross-sectional view taken along line II-II in FIG. 3, which shows the driving transistor of the unit pixel and the organic electroluminescence display device.
The organic electroluminescence device comprises a first electrode 307 on which an organic electroluminescence layer is formed and is connected to the second active layer 305, an organic electroluminescence layer 409 formed at a region defined by patterning a fifth insulating layer 407 formed on the fourth insulating layer 406, and a second electrode 408 formed on the organic electroluminescence layer 409.
As aforementioned, since the organic electroluminescence display device is provided with a plurality of thin film patterns, a number of photolithography processes are used which increases the cost of the organic electroluminescence display device.